1999 jimmy cycles form hot air to cool air constantly

Low freon for the summer, and low coolant for the winter. The heat and cooling are not related to each other. Separate systems

Suspect temperature control servo If stripping noise is heard when changing from hot to cold and back,replace servo.

Suggestion: see if the blend door opens or closes completely to allow all air through the heater core. To see the blend door moving, remove the panel that says "Cut here for service" under the hood along the firewall. I have a 99 Bravada where the blend door keeps moving back and forth constantly. Changed the blend door actuator but still keeps moving. The air inside is only warm since the door keeps allowing bypassed air through.

There is a flapper inside the duct work behind the dash that is getting stuck or has a bad connection. It is cycling back and forth, re-routing the air flow. It is not an easy fix and should be referred to a professional!

check the fuse and the wires.

your transmission oil is likely burnt,there by losing it's viscosity. check for brown rather than red fluid on dipstick.have transmission oil & filter changed.it is possible overheating will have caused additional damage.

get diagnostic-trouble code will tell you what is happening.

suspect failed relay to close flapper to swtich between heater and AC

You need to have the temperature valve actuator replaced. It's a dealer only part on top of a huge labor fee (about 600). I've been just opening my windows in the winter when it gets too hot... but with spring/summer coming I will have to have it rigged to constantly blowing cool air (and bring a sweater!). Good luck.

I WAS TOLD IN THE PAST TO GET RID OF THE ORANGE ANTIFREEZE AND PUT IN THE GREEN IN ALL MY VEHICLES CREATES MORE PROBLEMS THE IT CURES THIS IS BY A SHOP I TRUST.I HAVE A 95/98 AND 99 BLAZERS.I HAD PROBLEM WITH VERY LITTLE HEAT AFTER CHANGING THERMOSTAT. HAD TO GET AIR OUT OF LINES BUY RUNNING IT FROM COLD START WITH RADIATOR CAP OFF UNTIL THERMOSTAT OPENED AND LEFT IT RUNNING UNTIL I SAW NO AIR BUBBLES COMING OUT OF RADIATOR SEEMED TO CURE PROBLEM

check the blend door motor under (behind) the glovebox. make sure the ac system is cooling first.